The prior art is described in French patent application FR-A-2 089 639.
United States patent U.S. Pat. No. 5,401,476, which is hereby incorporated by reference, describes a combination of separation by simulated counter-current adsorption and crystallization to produce high purity para-xylene in a more economical manner than in a single step. The basic principles of that combination are as follows:
The simulated moving bed separation step is controlled in a different way to that for the direct production of high purity para-xylene. The feed flow rate is such that the flow rates in zone 2 and zone 3 (feed flow rate=flow rate in zone 2-flow rate in zone 3) cannot simultaneously produce an extract containing pure para-xylene and a raffinate which is free of para-xylene. The feed flow rate (and thus the productivity) is increased by reducing the flow rate in zone 2. It is thus impossible to obtain pure para-xylene (+than 98%). Further, the solvent flow rate is also reduced with respect to a "high purity" system in particular by increasing the flow rate in zone 4 above a threshold. The increase in productivity and the reduction in operating costs linked to the use of less solvent is to the detriment of purity.
In the crystallization step, a mixture containing 75% to 98% of para-xylene, more advantageously 85% to 95% of para-xylene, is treated to re-use pre-existing crystallisers in units based on crystallization schemes in two principal steps. A crystallization temperature which is preferably -15.degree. C. to +15.degree. C. is used so as not to consume frigories at a low heat level. In the most favourable cases, there is an additional synergy in using the same solvent as the desorbent in the adsorption step and as a rinsing solvent for the crystals in the crystallization step, which is different to the case for direct production of high purity para-xylene alone. However, when the feed adsorption step is carried out on the adsorbent with a high desorbent to feed ratio, for example over 1.3, very high purity of the desired product is obtained, in general over 95%. The higher the purity of the product, the more it can fluctuate. It is thus easy to imagine that when the purification step by crystallization, which can produce over 99% purity, is calculated for a feed of set purity which does not fluctuate, the operation of the crystalliser is disturbed.
U.S. Pat. No. 3,734,974 describes the use of X or Y zeolites exchanged with cations from group IA and/or group IIA (or a combination of the 2) with a controlled amount of water in the zeolite (1% to 5%) for the separation of para-xylene in a simulated moving bed (presumed to be high purity in the absence of any mention of low purity para-xylene). The desorbents used are, for example, toluene, or para-diethylbenzene or diethylbenzenes as a mixture or a mixture of these constituents with a paraffinic cut. Adding water, in particular to K BaX and BaX zeolites, brings about a very significant improvement of para-xylene-ethylbenzene and para-xylene-meta-xylene selectivities.
U.S. Pat. No. 4,778,946 describes the use of KY zeolite containing up to 10% of water and up to 4% of either methanol or ammonia for the separation of ethylbenzene and meta-xylene from feeds which are free of para-xylene so as not only to maximise the ethylbenzene-meta-xylene selectivity but also to obtain a desorbent-ethylbenzene selectivity as close as possible to 1. The desorbent/feed ratio used is 2/1. This document specifies that a desorbent which is too strongly adsorbed does not allow good separation and a desorbent which is too weakly adsorbed causes too excessive a demand for desorbent. It recornmends that the ethylbenzene-meta-xylene selectivity should be at least 3.0 and that the ethylbenzene-desorbent selectivity is in the range 1 to 2.
U.S. Pat. No. 5,401,476 does not suggest operating in the presence of water, as the hydrocarbons are anhydrous.
U.S. Pat. No. 3,734,974 does not suggest the possible importance of injecting water into a system where high purity para-xylene is not desired and does not recognise the role which water could play on the para-xylene-desorbent selectivity.
U.S. Pat. No. 4,778,946 teaches that water can modify the ethylbenzene-desorbent selectivity only in the case of feeds which are free of para-xylene. Neither U.S. Pat. No. 3,734,974 nor U.S. Pat. No. 4,778,946 suggests a simple and practical means not of controlling the water content in the zeolite but rather of aiming to control the water content in the hydrocarbons in contact with the zeolite.